The present invention relates to a circuit improvement for television receivers and, more particularly, to a circuit arrangement for sampling a particular line which is included at some appropriate vertical blanking interval.
A composite synchronizing signal, as shown by a waveform (a) in FIG. 1, includes horizontal sync pulses P.sub.1, equlizing pulses P.sub.2 and vertical sync pulses P.sub.3. All of the pulses have the same amplitude but they differ in frequency and pulse width. Various lines defined between the two neighboring horizontal sync pulses P.sub.1 and included within the vertical blanking interval are available for carrying various kinds of signals. For example, one type of signal is a reference signal which is commonly referred to as a vertical interval reference (VIR) signal. The VIR signal, as schematically shown in the waveform (a) in FIG. 1, is carried on line 19 of the televised image, or it could be included on some other line, or perhaps on a plurality of lines. The line in the vertical blanking interval may carry some other signals such as multiplexed voice signal and/or still picture signal. The description hereinbelow is particularly directed to a case where the line 19 carries the VIR signal.
The VIR signal, as illustrated graphically on an enlarged scale in FIG. 2 is used in the color television receiver set for automatic hue and saturation control. The VIR signal includes color burst component, chroma reference component, luminance reference component and black reference component. In order to utilize the VIR signal, it is necessary to detect and sample the particular line, that is, the line 19, carrying the VIR signal, and then, the VIR signal is detected for the comparison with the video signal.
In the step of sampling the particular line carrying the VIR signal, it is common practice to count pulses upto the particular line from a certain distinguished reference line which occurs prior to the particular line. According to one prior art, the reference line is distinguished by employing an integrating circuit having a time constant long compared with the duration of the equalizing pulse or horizontal sync pulse but not with respect to the vertical pulse width. Such integrating circuit is an RC integrating circuit in which the capacitor starts charging pulse voltage from the beginning of each field of even fields and odd fields to provide the waveform separation needed for vertical synchronization. The charged voltage across the capacitor is shown by waveforms (b) and (d) of FIG. 3 representing the even field and odd field, respectively.
When the horizontal or equalizing pulses are applied to the RC integrating circuit, they cannot be charged on the capacitor to any appreciable voltage because of the short duration of the pulsating period or because of the long time interval between the neighboring pulses.
However, when the vertical sync pulse is applied, the voltage across the capacitor can build up to a value required for triggering the circuit for detecting the reference line and/or other circuits such as vertical deflection oscillator.
Because there is a half-line difference in time between the even and odd fields (see waveforms (a) and (c) in FIG. 3), the odd field has the time interval between a first occuring equalizing pulse P.sub.1 ' and a final horizontal sync pulse P.sub.2 ' of a previous field shorter than that of the even field. Therefore, the capacitor establishes the triggering voltage more rapidly in the case of odd field than in the case of even field. Thus, the triggering moment in the odd field and that in the even field do not coincide with each other. As a consequence, the reference line detected by the RC integrating circuit may vary with respect to the difference of the field. In other words, the particular line carrying the VIR signal can be detected and sampled only in one field, even field or odd field.
Furthermore, since the charged voltage across the capacitor shows a serration, the triggering moment is apt to deviate to cause an erroneous function. Moreover, there is such a disadvantage that the RC integrating circuit requires an accurate adjustment for obtaining a predetermined time constant.
Prior art color television receivers dealing with the VIR signal containing television information, as described above, are disclosed, for example, in the U.S. Pat. Nos. 3,456,068, patented on July 15, 1969; 3,780,218, patented on Dec. 18, 1973; and 3,950,780, patented on Apr. 13, 1976.